When 4A is enabled, the softmixer binding depends on snd-aloop. This driver must be always present and inserted at the same place in the soundcards list. So having it builtin in the kernel is mandatory.

Currently on FF.rc2, the configuration doesn't seem homogeneous:

Correct setup on M3:

m3ulcb:~# zcat /proc/config.gz | grep -i ALOOP
CONFIG_SND_ALOOP=y

Bad setup on Qemu:

qemux86-64:~# zcat /proc/config.gz | grep -i ALOOP
CONFIG_SND_ALOOP=m

All machine builds should be checked (the script ikconfig located in the kernel tree may help to check the option on a non-running kernel binary)